Life cycles

To understand the forces determining the abundance of a population, we need to know the phases of the constituent organisms' lives when these forces act most significantly. For this, we need to understand the sequences of events that occur in those organisms' life cycles. A highly simplified, generalized life history (Figure 4.6a) comprises birth, followed by a prereproductive period, a period of reproduction, perhaps a postreproductive period, and then death as a result of senescence (though of course other forms of mortality may intervene at any time). The variety of life cycles is also

reproduction reproduction to senescence Time-►

Juvenile

J

Year 1

Time

Year 1

Time

Year 1 Year 2 Year 3 Year 4 Year 5

Year 1 Year 2 Year 3 Year 4 Year 5

Figure 4.6 (a) An outline life history for a unitary organism. Time passes along the horizontal axis, which is divided into different phases. Reproductive output is plotted on the vertical axis. The figures below (b-f) are variations on this basic theme. (b) A semelparous annual species.

(c) An iteroparous annual species.

(d) A long-lived iteroparous species with seasonal breeding (that may indeed live much longer than suggested in the figure).

(e) A long-lived species with continuous breeding (that may again live much longer than suggested in the figure).

(f) A semelparous species living longer than a year. The pre-reproductive phase may be a little over 1 year (a biennial species, breeding in its second year) or longer, often much longer, than this

summarized diagrammatically in Figure 4.6, although there are many life cycles that defy this simple classification. Some organisms fit several or many generations within a single year, some have just one generation each year (annuals), and others have a life cycle extended over several or many years. For all organisms, though, a period of growth occurs before there is any reproduction, and growth usually slows down (and in some cases stops altogether) when reproduction starts.

Whatever the length of their life cycle, species may, broadly, be either semelparous or iteroparous (often referred to by plant scientists as monocarpic and polycarpic). In semelparous species, individuals have only a single, distinct period of reproductive output in their lives, prior to which they have largely ceased to grow, during which they invest little or nothing in survival to future reproductive events, and after which they die. In iteroparous species, an individual normally experiences several or many such reproductive events, which may in fact merge into a single extended period of reproductive activity. During each period of reproductive activity the individual continues to invest in future survival and possibly growth, and beyond each it therefore has a reasonable chance of surviving to reproduce again.

For example, many annual plants are semelparous (Figure 4.6b): they have a sudden burst of flowering and seed set, and then they die. This is commonly the case among the weeds of arable crops. Others, such as groundsel (Senecio vulgaris), are iteroparous (Figure 4.6c): they continue to grow and produce new flowers and seeds through the season until they are killed by the first lethal frost of winter. They die with their buds on.

There is also a marked seasonal rhythm in the lives of many long-lived iteroparous plants and animals, especially in their reproductive activity: a period of reproduction once per year (Figure 4.6d). Mating (or the flowering of plants) is commonly triggered by the length of the photoperiod (see Section 2.3.7) and usually makes sure that young are born, eggs hatch or seeds are ripened when seasonal resources are likely to be abundant. Here, though, unlike annual species, the generations overlap and individuals of a range of ages breed side by side. The population is maintained in part by survival of adults and in part by new births.

In wet equatorial regions, on the other hand, where there is very little seasonal variation in temperature and rainfall and scarcely any variation in photoperiod, we find species of plants that are in flower and fruit throughout the year - and continuously breeding species of animal that subsist on this resource (Figure 4.6e). There are several species of fig (Ficus), for instance, that bear fruit continuously and form a reliable year-round food supply for birds and primates. In more seasonal climates, humans are unusual in also breeding continuously throughout the year, though numbers of other species, cockroaches, for example, do so in the stable environments that humans have created.

Amongst long-lived (i.e. longer than annual) semelparous plants (Figure 4.6f), some are strictly biennial - each individual takes two summers and the intervening winter to develop, but has only a single reproductive phase, in its second summer. An example is the white sweet clover, Melilotus alba. In New York State, this has relatively high mortality during the first growing season (whilst seedlings were developing into established plants), followed by much lower mortality until the end of the second summer, when the plants flowered and survivorship decreased rapidly. No plants survive to a third summer. Thus, there is an overlap of two generations at most (Klemow & Raynal, 1981). A more typical example of a semelparous species with overlapping generations is the composite Grindelia lanceolata, which may flower in its third, fourth or fifth years. But whenever an individual does flower, it dies soon after.

A well-known example of a semelparous animal with overlapping generations (Figure 4.6f) is the Pacific salmon Oncorhynchus nerka. Salmon are spawned in rivers. They spend the first phase of their juvenile life in fresh water and then migrate to the sea, often traveling thousands of miles. At maturity they return to the stream in which they were hatched. Some mature and return to reproduce after only 2 years at sea; others mature more slowly and return after 3, 4 or 5 years. At the time of reproduction the population of salmon is composed of overlapping generations of individuals. But all are semelparous: they lay their eggs and then die; their bout of reproduction is terminal.

There are even more dramatic examples of species that have a long life but reproduce just once. Many species of bamboo form dense clones of shoots that remain vegetative for many years: up to 100 years in some species. The whole population of shoots, from the same and sometimes different clones, then flowers simultaneously in a mass suicidal orgy. Even when shoots have become physically separated from each other, the parts still flower synchronously.

In the following sections we look at the patterns of birth and death in some of these life cycles in more detail, and at how these patterns are quantified. Often, in order to monitor and examine changing patterns of mortality with age or stage, a life table is used. This allows a survivorship curve to be constructed, which traces the decline in numbers, over time, of a group of newly born or newly emerged individuals or modules - or it can be thought of as a plot of the probability, for a representative newly born individual, of surviving to various ages. Patterns of birth amongst individuals of different ages are often monitored at the same time as life tables are constructed. These patterns are displayed in fecundity schedules.

semelparous and iteroparous life cycles the variety of life cycles

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